index.js contains the root node we create and mentions the document variable. There will be little to no changes to this file while writing the app, except maybe to import another Sass file.App.jsx contains the React app. This structure is to enable Fast Refresh (described below) but is also to completely separate the code mentioning the document variable from the actual React code for modularity.watch command to run Webpack, and have integrated watch functionality into Express. In index.mjs we detect that we are in the development environment and start the Webpack watcher from within Express. This means we can run both Express and Webpack simultaneously in watch mode with nodemon index.mjs.